Even though we make fun of Wikipedia, and even though any serious scientific piece shouldn't cite Wikipedia, fact remains that the community-created and maintained encyclopaedia has turned into an impressive database of knowledge. Even though I don't think you should trust it blindly, it's usually an excellent starting point for information, especially when used in a casual setting. Still, its open nature is also a threat to Wikipedia, this week exemplified by the Wikipedia Arbitration Committee banning Scientology from editing Wikipedia pages.